1. Field of the Invention
The present invention relates to a phase synchronization circuit used for generating a clock signal from an input signal.
2. Description of the Related Art
In recent years, a phase synchronization circuit has been used to generate a clock signal from an input signal. For example, a clock signal is generated, by using the phase synchronous circuit, from an input data signal read by a magnetic head in a magnetic disk drive.
Generally, the phase synchronization circuit is comprised of a phase comparator, a loop-filter, and a VCO (voltage-controlled oscillator). The phase comparator compares the phase of an output signal of the VCO with that of an input signal. An output signal of the phase comparator is provided to the loop filter, which provides a control voltage to the VCO. The loop filter may have various structures depending on the arrangement of the phase comparator.
In the loop filter having a charge pump circuit, a capacitor, and a resistor, the charge pump circuit controls the charging and discharging of the capacitor in response to an output signal of the phase comparator whose pulse width corresponds to a phase difference between the output signal of the VCO and the input signal. A charge voltage on the capacitor serves as the control voltage for the VCO. Since this control voltage controls the phase (frequency) of the output signal of the VCO, the phase of the output signal of the VCO is synchronized with that of the input signal.
The input signal is formed, for example, by reading a magnetic disk with a head of a magnetic disk drive and by shaping the read signal according to zero-crossing points and peaks of the signal. The VCO provides a clock signal whose phase is in synchronization with that of the input signal. According to the clock signal, the signal read through the head is processed to reproduce data.
By the way, some magnetic disk drives employ a zone bit recording method that arranges more sectors in outer tracks than in inner tracks to increase storage capacity. According to this method, the clock frequency used when accessing inner tracks differs from that used when accessing outer tracks.
When generating a clock signal with the phase synchronization circuit in a magnetic disk unit employing the zone bit recording method, the frequency of an input signal may fluctuate, and the output signal of the VCO must follow such fluctuations. Accordingly, the charge pump circuit may change the charging or discharging current of the capacitor. This results in changing the damping factor of the phase synchronization loop, and this destabilizes the phase synchronization operation.
In order to generate a stabilized clock signal with such fluctuating clock frequencies, it is necessary for the phase synchronization circuit to provide a plurality of loop filters for the different frequencies, and to select a suitable loop filter by switching in accordance with the frequency. This may cause the construction of the phase synchronization circuit to be complicated and to increase the cost of the circuit.